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Message started by Gus on 10/23/15 at 11:34:05

Title: backfire
Post by Gus on 10/23/15 at 11:34:05

Backfires through exhaust when letting up on throttle unless you let up slowly. Not near as pronounced if you leave the choke on part way.
I tried octane booster. Maybe helped just a smidge but not much. To lean, obviously, is there any way to adjust the carb. I've heard not.

Thanks,
Gus

Title: Re: backfire
Post by Dave on 10/23/15 at 12:02:36

This is not the motorcycle that you should be closing the throttle quickly. Doing so is certain to make noise when you do that, and putting on a muffler that is less restrictive than the stock one will make it even more pronounced. Carb jetting might make it a bit better - but it will not stop it from happening - and you should jet to make your engine run well.....you should not be jetting your bike coast quietly. If you jet the bike really rich in the pilot jet circuit you can cut back on the noise - it will make the engine run too rich at low speeds. You need to adjust your riding style to suit the big, primitive single with a carb.....this is not a modern, fuel injected motorcycle - it is a primitive beast.

When you let the throttle snap close, you have created a high engine vacuum that pulls a lot of air past the butterfly and slide - but the main and needle jets are closed off....only the idle circuit is providing fuel. The fuel mixture goes very lean as a result, and the spark plug can not ignite the mixture in the cylinder - but when the lean mixture builds up in the exhaust it can ignite and make the popping and banging sounds. If you jet to eliminate the noise when coasting - the engine will be running too rich when at a normal idle (same thing when pulling out the choke).

When you shift gears you should gently roll the throttle off - but stop just a bit short of being all the way closed. Same thing when you are coasting down a hill or slowing down in traffic - roll the throttle off just far enough to slow down - but not all the way closed or the noise will begin.

It is all part of being a "good" rider - you need to adapt your style to suit the motorcycle and it's quirks.

Title: Re: backfire
Post by norm92de on 10/23/15 at 12:57:11

I had backfiring on deceleration. Adjusting the idle mixture screw completely cured it. I still have an after fire on shutdown sometimes, but as Versy said it is just a belch of power.

I also raised the needle .035" to help pickup off idle. The overall mixture seems to be quite close right now at 4000'. I didn't address the main jet size yet but I don't ride on the main very much. Winter is coming on so maybe it will be richer in the colder weather.

Title: Re: backfire
Post by norm92de on 10/23/15 at 12:59:16

Leaner in the colder weather :-[

Title: Re: backfire
Post by norm92de on 10/23/15 at 13:05:18

Talking about mixture. On aircraft we used to lean out the engine while taxying to the runway to avoid fouling the plugs. If you didn't the mag drop was usually excessive, especially on Lycomings. :)

Title: Re: backfire
Post by Gus on 10/23/15 at 13:46:19

Ok Norm that is info. I can use. Awesome.

Gus

Title: Re: backfire
Post by Gus on 10/23/15 at 13:52:52

I know my old shovel head demanded a carb adjustment she the weather cooled off and again in the spring. Couple older dirt singles I've had too. I also had an old 1957 Jeep pickup when I was a kid that required semi annual carb and timing adjustments. It was actually kind of fussy about altitude too.

Gus

Title: Re: backfire
Post by Kris01 on 10/23/15 at 17:51:41

Temperature, altitude and humidity will affect the carb settings. Less air = less gas; more air = more gas. ;)

Title: Re: backfire
Post by marekbuk on 10/24/15 at 04:43:32

My original pilot jet size 52.5 would only tune with 3.5 turns out of mixture screw so fitted 55 jet and hardly any difference on mixture screw - approx. 3 turns out but lots of backfiring even on gear changes.

Tried a 57.5 yesterday and now the mixture tunes at 2 turns out but the bike stalled on throttle off so had to set a fast idle to stop this happening and lots of backfiring still.

Going to go back to original 52.5 with 3.5 turns out as it ran ok like that with very few backfires.

Seems the Brit Savages are pretty forgiving of fitting the Harley silencers with existing jetting.

Title: Re: backfire
Post by Gus on 10/24/15 at 06:19:23

Other than aftermarket mufflers and silencers; I wonder if anyone has experimented with header length at all?

Title: Re: backfire
Post by verslagen1 on 10/24/15 at 08:12:33

search out the works of diamond jim.

Title: Re: backfire
Post by Kris01 on 10/24/15 at 19:50:20

Correct me if I'm wrong but header length only changes the power curve. It shouldn't affect backfiring, smoothness, etc.

Title: Re: backfire
Post by Gus on 10/24/15 at 20:09:59

You got me there; I wouldn't know. Just thought it may be something to consider. Would changing the power curve affect the amount of vacuum produced by the engine? According to what Dave was saying part of the reason for the backfire is the unusual amount of vacuum produced by the engine. I know that it is just a part of it but maybe; just maybe. . .

744D564C0F0E3F0 wrote:

Correct me if I'm wrong but header length only changes the power curve. It shouldn't affect backfiring, smoothness, etc.

Title: Re: backfire
Post by Dave on 10/25/15 at 04:57:11

My comment about the high vacuum was in a PM to Gus, and the condition I was referring to is when you close the throttle and the engine is slowing down with only the Pilot circuit providing fuel.

The fuel mixture goes really lean when the throttle is closed and the engine is either slowing down between shifts - or you are slowing down and have the throttle fully closed. There is a high engine vacuum - but with the throttle closed and the slide down...the only fuel that is getting to the engine is coming from the idle circuit....and that is not enough fuel to allow the spark plug to ignite the mixture. So the fuel/air in the cylinder is not ignited and it just builds up in the exhaust....and eventually the mixture reaches a high enough temperature in the hot exhaust pipe that it spontaneously ignites and makes a bang.

Suzuki tried to cure the problem with the TEV (Throttle Enrichment Valve) which is supposed to allow more fuel to flow during those high vacuum events - but it doesn't seem to work well with the E10 fuel. Maybe it works OK for folks who can still find pure gasoline. Some folks have fiddled with the spring in the TEV and claim to have made things better - but I don't feel it is a issue with the TEV not operating soon enough - I believe it just doesn't add enough E10 fuel.

There are folks who put in larger pilot jets and open the idle mixture screw, and this helps compensate for the lean mixture when decelerating.....at the expense of an overly rich mixture when the bike is idling or at low throttle settings. Fuel mileage can suffer considerably, and the engine can belch black smoke when idling, and the engine can get loaded up with carbon.

You really can make a big difference in the amount of backfire by how you use your right hand. When you shift gears just roll the throttle slightly back and don't let it close completely between shifts. When coasting just open the throttle a tiny bit until the backfire goes away - but not so much that the bike is accelerating....just enough that the slide rises and allows more fuel into the engine.

Title: Re: backfire
Post by Kris01 on 10/25/15 at 07:05:17

Dave is right. I've tried that technique and it works.

Also, to "remove" the shutdown bang, you can ease off the clutch while holding the brake until the engine dies. Apparently the idle slows to the point that the engine isn't pulling enough vacuum for the carb to draw any fuel. No fuel = no afterfire.

But, if you're like me, I like to watch the end of the exhaust at night when I shut her down. I get a small pop and a little fireworks show. ;D

Title: Re: backfire
Post by Gus on 10/25/15 at 07:31:29

Dave is right about the easing of the throttle. After riding this bike for a couple days I realized that with less aggressive, thoughtful throttle control the pop isnt near as pronounced.I just thought there may have been a successful remedy to eliminate it all together.

122B302A6968590 wrote:

Dave is right. I've tried that technique and it works.

Title: Re: backfire
Post by MnSpring on 10/26/15 at 10:00:14

Rather new at this ICE in the ls650/S40.

I also had the, 'backfire'.
Riding was little problem, could be controlled.
It was Shut Down.
(Especially under the overhang at the bank drive through. And the P.O, and Cafe)

Understood the rapid, 'shut down' (riding) and the, 'big single' vacuum. Dealt with that. But the, 'shut down', "BANG".

I Believe it was, air.
Found the header bolts were loose.
And the clamp, (header to factory muffler), also loose.
Did some stuff, and reduced the, "BANG" to a 'poof'.
And if I let it idle, for about 20-30 sec, no 'poof'.

So perhaps try, 'air', leaking into the exhaust system somewhere ?
(The thing I have to constantly fiddle with is that, 'clamp', muffler over header)

So now, when the, 'Bang' comes back, a little maintenance, and it is gone again. But in the meantime, (when I want it quiet), It's, as others have said, Brakes, Gear, slowly let out clutch. Or, pull the choke, then shut off. Although it probably doesn't hurt, (just like giving a dog a treat when not training, it makes, You, feel good). I don't like putting the wash of extra gas, on top of the piston.

Title: Re: backfire
Post by Kris01 on 10/26/15 at 15:57:05

A word of caution on tightening the header bolts (ok, maybe a couple of words): They are pretty fragile and will break. Use a 1/4" ratchet and a light touch. You just want to snug them down, not crank them down to 100 ft/lbs.

Title: Re: backfire
Post by MnSpring on 10/26/15 at 17:47:21

69504B511213220 wrote:

YES, ANY, Steel bolt, in a Aluminum hole.

The header. One was hand loose, the other just touched the wrench.

Took it off, Cleaned the gasket, flat sanded the pipe.
(Block of wood, 400 grit W/D paper)
Anti seize on the bolts, and both sides of the gasket.
Put back in, with, ‘feel’. Didn’t have a torque wrench that small.

Those two bolts, have not come loose.
It’s that darn ‘clamp’ between the header & muffler,
that always causes the problems.

Title: Re: backfire
Post by KennyG on 10/26/15 at 17:55:49

MN,

When I was having trouble with a leak where the header, adapter and muffler were joined I ended up using Permatex exhaust sealer.

http://www.amazon.com/gp/product/B000BOAZM8?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00

Kenny G

Title: Re: backfire
Post by Kris01 on 10/27/15 at 20:00:11

I covered the joint in high temp silicone and then wrapped it in aluminum foil before I clamped it. So far no leaks.